Stormwater is pure rainwater plus any particulate debris and dissolved materials that the rainwater carries along with it. In urban areas, rain that falls on the roofs of houses, collects on paved areas like driveways, roads and sidewalks is typically diverted through a system of pipes that is separate from the sewerage system. Unlike sewage, stormwater was historically not treated, but flowed directly from streets and gutters into rivers, lakes and the ocean.
Stormwater can therefore be a form of diffuse or non-point source pollution. It can entrain pollutants, such as garbage, sediment, organic matter, heavy metals, and organic toxins, and flush them into receiving water bodies. As a consequence, natural bodies of water that receive stormwater may also receive pollutants capable of irreparable environmental harm.
The amount of stormwater pollution entering into such receiving bodies of water is related to the degree of urbanization in the surrounding area and the nature of the surrounding activities. Urbanization results in the covering of land with low-permeability structures, such as roadways, parking lots, and rooftops, which both generate large volumes of stormwater and accumulate pollutants. Since these types of surfaces do not allow rainfall to infiltrate, they allow the accumulated pollutants to be washed into stormwater drainage systems.
Prior to extensive human modification of the natural landscape, contaminated stormwater was not a significant problem. Rain fell on porous, natural surfaces where gradual percolation through the earth provided natural filtration before stormwater entered aquifers, streams, lakes, bays, and other natural water bodies. In addition, erosion due to excessive stormwater volumes was minimal and contaminants such as garbage, oil and grease, heavy metals, and organic chemicals were insignificant. In today's largely urban and industrial world, stormwater is a significant source of aquatic pollution.
In an effort to address the environmental problems posed by polluted stormwater, traps and filters for stormwater have been developed. U.S. Pat. No. 5,322,629 (hereby incorporated by reference) describes a method and apparatus for treatment of stormwater by vertical filtration through a bed of high-quality leaf compost material that removes pollutants prior to discharge into a receiving water body.
A granular form of compost that improves the overall performance of such filtration systems was described in U.S. Pat. No. 5,624,576 (hereby incorporated by reference). The permeability of a filter containing such granular compost is increased, and can be maintained for an extended period of time.
Extended filter permeability can be achieved by orienting the surface of the media bed vertically rather than horizontally, as described in U.S. Pat. No. 5,707,527 (hereby incorporated by reference). Vertical filter beds discourage the collection and storage of settled solids on the surface of the filter, which otherwise typically leads to “blinding” or surface loading, a rapid clogging of the outer surface of the filter. The use of a vertical filtration surface also facilitates the maintenance of the system both by keeping the bulk of the captured solids off of the media surface and by forcing the enclosure of the media into more manageable, engineered structures.
The improved filter apparatus described in U.S. Pat. No. 6,027,639 (hereby incorporated by reference) features a siphon-inducing mechanism and is somewhat “self-cleaning”, thereby improving the operational life of the media bed. The filter apparatus of the '639 patent utilized a sealed upper housing that included a check valve for air evacuation within the housing. An induced siphon effect pulled stormwater through the filter until air was pulled through an air inlet along the lower perimeter of the housing, disrupting the siphon. It was intended that the air stream that resulted from such a siphon-breaking event would produce turbulence and thereby clean the filter. In particular, the filter apparatus of the '639 patent utilized a fabric filtration medium around the outside of the media bed to prevent particulates from clogging the media bed, and the periodic turbulence was intended to keep the fabric filter free of clogging particulates.
Unfortunately, although the filter basket of the '639 patent was an improvement over existing stormwater filter systems, it nevertheless possessed some shortcomings. The filter assembly typically featured a flush, circumferential air inlet along the lower perimeter of the filter housing, and the air streams that resulted upon breaking the siphon were therefore typically concentrated along a single highest portion of the lower perimeter of the filter housing. The resulting localized air stream resulted in the cleaning of only a single section of the fabric filter medium, rather than the entire filter surface. In addition, it was found that fabric filter media often suffered from rapid clogging. Filtered particulate matter could create rapid and persistent surface loading of the fabric filter, which was then exaggerated by the effects of a poorly functioning cleaning mechanism.
An improved self-cleaning stormwater filter apparatus was described in U.S. application Ser. No. 09/990,564 for FILTER CARTRIDGE WITH REGULATED SURFACE CLEANING MECHANISM, filed Nov. 20, 2001. The improved filter apparatus included a hood that incorporated a plurality of voids arranged in a horizontally-aligned array. As an established siphon effect drew stormwater through this improved filter assembly, the water surface elevation outside the hood eventually dropped to the level of the void array, disrupting or “breaking” the siphon. The horizontally-aligned voids in the filter hood were able to direct and regulate the size of the air streams rising in the space between the hood and the filter medium, agitating the outer screen and the surfaces of the filter medium. These turbulent air streams were better able to dislodge particulate matter from the filter surfaces that would otherwise clog the filter and/or slow filtration.
The filter assemblies described previously could occasionally be fully submerged by stormwater during the service life of the cartridges. For example, complete submergence of the filter assembly may be desirable where the available volume above the filter assemblies can be used to detain and store stormwater prior to filtration. Additionally, a greater hydraulic driving head through the cartridge permits greater flexibility in cartridge media selection. For example, more effective filtration may be achieved through the selection of finer filter media, which may require a greater hydraulic driving head.
While the check valve in the upper housing of the filter assembly allows the air trapped under the cartridge hood to purge to atmosphere, complete submergence of the filter assembly may allow stormwater to flow into the valve cavity as well. Particulates and other debris in the stormwater can then settle on the check valve seat and prevent the valve from functioning properly. A compromised check valve could permit air to enter the filter hood, preventing the establishment of the requisite siphon effect entirely, potentially disrupting the self-cleaning of the filter medium.
What is needed is a stormwater filter apparatus that helps minimize the likelihood of the check valve and check valve seat being fouled with debris during operation, and yet does not interfere with the operation of the filter assembly, or contribute additional mechanical complexity to the filter assembly.